1<html lang="en"> 2<head> 3<title>If - The C Preprocessor</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="The C Preprocessor"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax"> 9<link rel="prev" href="Ifdef.html#Ifdef" title="Ifdef"> 10<link rel="next" href="Defined.html#Defined" title="Defined"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 141997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 152008, 2009, 2010, 2011 16Free Software Foundation, Inc. 17 18Permission is granted to copy, distribute and/or modify this document 19under the terms of the GNU Free Documentation License, Version 1.3 or 20any later version published by the Free Software Foundation. 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Its syntax is 65 66<pre class="smallexample"> #if <var>expression</var> 67 68 <var>controlled text</var> 69 70 #endif /* <var>expression</var> */ 71</pre> 72 <p><var>expression</var> is a C expression of integer type, subject to stringent 73restrictions. It may contain 74 75 <ul> 76<li>Integer constants. 77 78 <li>Character constants, which are interpreted as they would be in normal 79code. 80 81 <li>Arithmetic operators for addition, subtraction, multiplication, 82division, bitwise operations, shifts, comparisons, and logical 83operations (<code>&&</code> and <code>||</code>). The latter two obey the usual 84short-circuiting rules of standard C. 85 86 <li>Macros. All macros in the expression are expanded before actual 87computation of the expression's value begins. 88 89 <li>Uses of the <code>defined</code> operator, which lets you check whether macros 90are defined in the middle of an ‘<samp><span class="samp">#if</span></samp>’. 91 92 <li>Identifiers that are not macros, which are all considered to be the 93number zero. This allows you to write <code>#if MACRO<!-- /@w --></code> instead of 94<code>#ifdef MACRO<!-- /@w --></code>, if you know that MACRO, when defined, will 95always have a nonzero value. Function-like macros used without their 96function call parentheses are also treated as zero. 97 98 <p>In some contexts this shortcut is undesirable. The <samp><span class="option">-Wundef</span></samp> 99option causes GCC to warn whenever it encounters an identifier which is 100not a macro in an ‘<samp><span class="samp">#if</span></samp>’. 101</ul> 102 103 <p>The preprocessor does not know anything about types in the language. 104Therefore, <code>sizeof</code> operators are not recognized in ‘<samp><span class="samp">#if</span></samp>’, and 105neither are <code>enum</code> constants. They will be taken as identifiers 106which are not macros, and replaced by zero. In the case of 107<code>sizeof</code>, this is likely to cause the expression to be invalid. 108 109 <p>The preprocessor calculates the value of <var>expression</var>. It carries 110out all calculations in the widest integer type known to the compiler; 111on most machines supported by GCC this is 64 bits. This is not the same 112rule as the compiler uses to calculate the value of a constant 113expression, and may give different results in some cases. If the value 114comes out to be nonzero, the ‘<samp><span class="samp">#if</span></samp>’ succeeds and the <var>controlled 115text</var> is included; otherwise it is skipped. 116 117 </body></html> 118 119